1. Field of the Invention
The present invention relates to an alloy coating for use as a surface coating which can prolong the service life of members for apparatuses for high temperature applications (hereinafter referred to as “members for high temperature apparatuses”), such as gas turbine blades, jet engines, and heat-transfer tubes for boilers, a method for forming the same, and a member for high temperature apparatuses.
2. Description of the Related Art
For members for high temperature apparatuses, such as industrial gas turbine blades and heat-transfer tubes for boilers, in many cases, a coating is applied on the surface thereof to improve the heat resistance and the corrosion resistance of the members. In general, in order to improve the heat resistance, a ceramic coating called “thermal barrier coating” (hereinafter referred to as “TBC”) is applied to the surface of a substrate. Such a ceramic coating, however, has the problem that the difference in coefficient of thermal expansion between the substrate metal and the ceramic is so large that, when a ceramic layer is directly formed on the surface of the substrate, the ceramic layer is likely to be separated from the interface of the ceramic layer and the substrate. In order to solve this problem, in general, as shown in FIG. 3C, an undercoat 50 formed of an alloy layer and a topcoat 52 formed of, for example, a ceramic such as ZrO2 are laminated in that order on the surface of the substrate 10, to improve the adhesion of TBC 54 to the substrate 10.
Under very high-temperature environments of about 800 to 1200° C., however, as shown in FIG. 3C, the undercoat 50 is reacted with the substrate 10 to form an Al-deficient layer 56 and a layer 58 of Al2O3+NiAl2O4 having poor protective properties at the interface of the undercoat 50 and the substrate 10, thus resulting in deterioration of TBC 54. Further, since TBC 54 is porous, atmosphere gas enters the inside of TBC 54 and then internally oxidizes or internally nitrides the substrate 10 to form an internal corrosion layer containing an internal oxide 36 and an internal nitride 38 within the substrate 10. For this reason, the service life of the member for high temperature apparatuses is as short as several months. This is a severe problem involved such members having a conventional ceramic coating. In a prior art technique, an attempt has been made to use a Pt—Al spray coating after Pt plating, usually, to improve the heat resistance. This technique, however, cannot attain a satisfactory improvement.
On the other hand, in order to improve the corrosion resistance, coating techniques such as diffusion coating of Cr or Al and spray coating of a high Ni-high Cr alloy have been utilized. Such coatings, however, have the drawback that when the coated member is used under a very high-temperature environment of about 800 to 1200° C., the element contributing to the corrosion resistance is very rapidly diffused and, in addition, is highly reactive. Thus, the protective coating cannot be maintained stably for a long period of time. Further, even in the temperature range of 500 to 800° C., when the member is used in a strongly corrosive environment containing, for example, chlorine (Cl) or sulfur (S), the element constituting the protective coating, such as Cr or Al, is rapidly consumed. Thus, again, the protective coating cannot be stably maintained for a long period of time, resulting in a short service life of the apparatus. For the above reasons, at the present time, the prolongation of the service life of a member for high temperature apparatuses is only made by lowering the service temperature of the apparatus while sacrificing the performance of the apparatus.
As described above, in the case of members used at high temperatures, even when TBC is applied to the members to improve the heat resistance, the TBC layer is deteriorated as a result of a reaction of TBC with the substrate during use of the members, and, in addition, the substrate is internally oxidized or internally nitrided by atmosphere gas which has penetrated into the inside of the coating. Further, even with the application of the diffusion coating of Cr or Al, the spray coating of a high Ni-high Cr alloy, etc., for improving the corrosion resistance, when the apparatus is used under an environment containing Cl, S or the like, which is highly corrosive at high temperatures, the consumption rate of the element constituting the protective coating, such as Cr or Al, is so high that a stable protective coating cannot be maintained for a long period of time.
It has now been found that rhenium (Re), iridium (Ir), niobium (Nb), tantalum (Ta), molybdenum (Mo), and tungsten (W), which are high-melting metals, when alloyed with an element for imparting corrosion resistance, such as chromium (Cr), aluminum (Al), silicon (Si), magnesium (Mg), niobium (Nb), tantalum (Ta), nickel (Ni), cobalt (Co), iron (Fe), molybdenum (Mo), iridium (Ir), tungsten (W), platinum (Pt), or rhodium (Rh), to form an alloy phase, are stable at a high temperature of 1100° C. or above, even at 1150° C. or above, and, at the same time, possess excellent oxidation resistance. Further, as a result of studies on a high temperature corrosion reaction of a Ni-base superalloy coated with a Re (Ir, Nb, Ta, Mo, and W) —X alloy (wherein X=Cr, Al, Si, Mg, Nb, Ta, Ni, Co, Fe, Mo, Pt, Rh, Ir, W or the like), it has been found that an alloy phase containing Re (Ir, Nb, Ta, Mo, and W) can inhibit the outward diffusion of Ni, Al, Ti, Ta or the like and the inward diffusion of an oxidizing agent or the like. Thus, it is expected that provision of a thin layer of this alloy based on Re (Ir, Nb, Ta, Mo, and W) on the surface of a substrate can realize the inhibition of the outward diffusion of the alloying element from the substrate and the inward diffusion of the oxidizing agent or the like from the environment, which has been unsatisfactory with the conventional Pt—Al sprayed coating, leading to less corrosion and damage in the substrate whereby the service life of the member for high temperature apparatuses would be prolonged. Further, for members on which TBC is to be applied, when the above-described coating is interposed between the substrate and TBC, the interposed coating is considered to function as an excellent diffusion barrier which can inhibit the deterioration of TBC, caused by a reaction of TBC with the substrate, and the internal corrosion of the substrate.
As coating techniques used for improving heat resistance and corrosion resistance, PVD, CVD and spray coating are known. These methods have the drawbacks that: {circle around (1)} control of the thickness and composition of the coating is difficult; {circle around (2)} a large coating apparatus is needed, and the operation is complicated; {circle around (3)} the formed coating has many defects and cracks; {circle around (4)} there is a number of limitations on the size and shape of a substrate to be coated (for example, coating faithfully conforming to the shape of concaves and convexes is difficult); and {circle around (5)} the cost is high. In contrast, the technique of plating in a molten salt has the advantages that {circle around (1)} the thickness and composition of the coating can be easily controlled; {circle around (2)} the coating can be effected with ease by using a simple apparatus; {circle around (3)} a dense coating having no significant defect can be formed; {circle around (4)} there is few limitations on the size and shape of a substrate to be coated; and {circle around (5)} the cost is low.